1. Field of the Invention
The present invention relates generally to a protective circuit for a power amplifier, and is directed more particularly to such a protective circuit for a power amplifier so as to protect the power amplifier from being damaged by a high frequency oscillation signal.
2. Description of the Prior Art
Recently, a power amplifier superior in high frequency characteristic has been proposed whose slew rate (voltage changing ratio of an amplified output when a rectangular wave is applied to the power amplifier) is 400 to 500 V/.mu.sec, while that of a prior art power amplifier up to that time is 80 V/.mu.sec. Such power amplifier amplifies the high frequency components of a signal with high fidelity, so that a sound can be reproduced without disturbing the fine sound quality.
There is danger that an oscillation may be caused in such power amplifier at a high frequency range due to a stray capacity. If such high frequency oscillation appears, the power amplifier per se and also a speaker driven thereby are some times damaged. That is, when an oscillation is generated at a relatively low frequency range, since the oscillations waveform is disturbed, DC components are generated and hence a DC protective circuit is operated to be able to avoid the generation of oscillation in the prior art. However, when the oscillation frequency is high, for example, about 1 MHz, the oscillation waveform becomes approximately a sine wave and hence no DC component is produced. Accordingly, the power amplifier can not be protected from the high frequency oscillation by only the prior art DC protective circuit.
When an oscillation is caused at high frequency, since through a transistor serving as an amplifying element of the power amplifier, a current continuously flows due to the stored charge at a high frequency, for example, about 1 MHz (upon high frequency, an oscillation loop is established by small stray capacity), there is a danger that the power amplifier may be damaged. Further, at this time since a large power is applied to a speaker, the speaker is also damaged.
The above will be now considered with a case as an example where a power amplifier is connected to a system selector. This system selector is formed of as shown in FIG. 1 change-over switches 1 and 2 and is used in, for example, an audio specialty store. In FIG. 1, references 3.sub.1, 3.sub.2, . . . 3.sub.n designate tuners (or record players) whose output terminals are respectively connected to input terminals or contacts 1.sub.a1, 1.sub.a2, . . . 1.sub.an of the change-over switch 1. Output terminals or contacts 1.sub.b1, 1.sub.b2, . . . 1.sub.bn of this change-over switch 1 are respectively connected to input terminals of amplifiers 4.sub.1, 4.sub.2, . . . 4.sub.n whose output terminals are respectively connected to input terminals or contacts 2.sub.a1, 2.sub.a2, . . . 2.sub.an of the change-over switch 2 output terminals or contacts 2.sub.b1, 2.sub.b2, . . . 2.sub.bn of which are connected to speakers 5.sub.1, 5.sub.2, . . . 5.sub.n , respectively.
According to the above system selector, it is possible to connect the tuners 3.sub.1 to 3.sub.n, amplifiers 4.sub.1 to 4.sub.n and speakers 5.sub.1 to 5.sub.n in any desired combination by suitably switching over the change-over switches 1 and 2.
With such system selector, however, it is required frequently to have long connection cords and hence a stray capacity C.sub.F is formed as shown in FIG. 1 by the broken line resulting in the formation of an oscillation loop including the amplifiers 4.sub.1 to 4.sub.n. When the prior art amplifiers are used as the amplifiers 4.sub.1 to 4.sub.n, the oscillation frequency may be considered relatively low so that they can be of course protected by a DC protective circuit and so on. However, when the power amplifiers whose slew rate is as large as 400 to 500 V/.mu.sec are used as the amplifiers 4.sub.1 to 4.sub.n, oscillation is caused at high frequency and troubles occur frequently, which is practically noted at present.
In order to avoid the generation of such oscillation, it is important that the cord is not unnecessarily long. It is however, sometimes appeared that the cord is moved temporarily or carelessly. In such case, if the amplifiers 4.sub.1 to 4.sub.n and speakers 5.sub.1 to 5.sub.n are damaged, it is very inconvenient. Therefore, it is desired that when high frequency oscillation is caused, the amplifiers 4.sub.1 to 4.sub.n are made inoperative, and then the cord movement is made less so as to use the cord normally.